Vitamin D is an essential nutrient with important physiological roles in the positive regulation of calcium (Ca2+) homeostasis. Vitamin D can be made de novo in the skin by exposure to sunlight or it can be absorbed from the diet. There are two forms of vitamin D; vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D3 is the form synthesized de novo by animals. It is also a common supplement added to milk products and certain food products produced in the United States. Both dietary and intrinsically synthesized vitamin D3 must undergo metabolic activation to generate bioactive metabolites. In humans, the initial step of vitamin D3 activation occurs primarily in the liver and involves hydroxylation to form the intermediate metabolite 25-hydroxyvitamin D3 (25-hydroxycholecalciferol; calcifediol; 25OHD3). Calcifediol is the major form of vitamin D3 in the circulation. Circulating 25OHD3 is then converted by the kidney to 1,25-dihydroxyvitamin D3 (calcitriol; 1,25(OH)2D3), which is generally believed to be the metabolite of vitamin D3 with the highest biological activity.
Vitamin D2 is derived from fungal and plant sources. Many over-the-counter dietary supplements contain ergocalciferol (vitamin D2) rather than cholecalciferol (vitamin D3). Drisdol, the only high-potency prescription form of vitamin D available in the United States, is formulated with ergocalciferol. Vitamin D2 undergoes a similar pathway of metabolic activation in humans as vitamin D3, forming the metabolites 25-hydroxyvitamin D2 (25OHD2) and 1,25-dihydroxyvitamin D3 (1,25(OH)2D2). Vitamin D2 and vitamin D3 have long been assumed to be biologically equivalent in humans, however recent reports suggest that there may be differences in the bioactivity and bioavailability of these two forms of vitamin D (Armas et. al., (2004) J. Clin. Endocrinol. Metab. 89:5387-5391).
Measurement of vitamin D, the inactive vitamin D precursor, is rare in clinical settings and has little diagnostic value. Rather, serum levels of 25-hydroxyvitamin D3 and 25-hydroxyvitamin D2 (total 25-hydroxyvitamin D; “25OHD”) are a useful index of vitamin D nutritional status and the efficacy of certain vitamin D analogs. Therefore, the measurement of 25OHD is commonly used in the diagnosis and management of disorders of calcium metabolism. In this respect, low levels of 25OHD are indicative of vitamin D deficiency associated with diseases such as hypocalcemia, hypophosphatemia, secondary hyperparathyroidism, elevated alkaline phosphatase, osteomalacia in adults and rickets in children. In patients suspected of vitamin D intoxication, elevated levels of 25OHD distinguishes this disorder from other disorders that cause hypercalcemia.
Measurement of 1,25(OH)2D is also used in clinical settings, however, this metabolite has a more limited diagnostic usefulness than 25OHD. Factors that contribute to limitations of the diagnostic values of 1,25(OH)2D as an index of vitamin D status include the precision of the endogenous regulation of renal production of the metabolite and its short half-life in circulation. However, certain disease states such as kidney failure can be diagnosed by reduced levels of circulating 1,25(OH)2D and elevated levels of 1,25(OH)2D may be indicative of excess parathyroid hormone or may be indicative of certain diseases such as sarcoidosis or certain types of lymphoma.
Detection of vitamin D metabolites has been accomplished by radioimmunoassay with antibodies co-specific for 25-hydroxyvitamin D3 and 25-hydroxyvitamin D2. Because the current immunologically-based assays do not separately resolve 25-hydroxyvitamin D3 and 25-hydroxyvitamin D2, the source of a deficiency in vitamin D nutrition cannot be determined without resorting to other tests. More recently, reports have been published that disclose methods for detecting specific vitamin D metabolites using mass spectrometry. For example Yeung B, et al., J Chromatogr. 1993, 645(1):115-23; Higashi T, et al., Steroids. 2000, 65(5):281-94; Higashi T, et al., Biol Pharm Bull. 2001, 24(7):738-43; and Higashi T, et al., J Pharm Biomed Anal. 2002, 29(5):947-55 disclose methods for detecting various vitamin D metabolites using liquid chromatography and mass spectrometry. These methods require that the metabolites be derivatized prior to detection by mass-spectrometry. Methods to detect underivatized 1,25(OH)2D3 by liquid chromatography/mass-spectrometry are disclosed in Kissmeyer and Sonne, J Chromatogr A. 2001, 935(1-2):93-103.